2.1. Synthesis
The synthesis of the rod-like mesogens mainly involves three key steps including Suzuki coupling reaction, etherification reaction and Knoevenagel condensation reaction as shown in Scheme 1. Firstly, Suzuki coupling reaction pyren-1-ylboronic acid and 4-bromobenzaldehyde or 2-(4-bromophenyl)acetonitrile took place to yield compounds 1.[31,32] In addition, 4-hydroxybenzaldehyde and 2-(4-hydroxyphenyl)acetonitrile were o-alkylated with bromotetradecane in the presence of base to yield compounds 2. Finally, compounds 1 were reacted with compounds 2 under the Knoevenagel condensation condition to yield the rod-like liquid crystals BI-CN-4 and BI-CHO-4. The synthetic details and corresponding spectrum analysis results were provided in the Supplemental Materials.
2.2. Liquid-crystalline properties
The thermal behavior and optical textures of compounds BI-CN-4 and BI-CHO-4 were studied by using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). On heating process of compound BI-CN-4 under POM, the crystal transformed into the isotropic state at about 175 ℃. On first cooling process from isotropic state of compound BI-CN-4, irregular schlieren texture could be observed indicating that compound BI-CN-4 could self-organize into smectic C phase (Fig. 1a).[33] On heating process of compound BI-CHO-4 under POM, the crystal gradually transformed into mesophase at about 144 ℃ and then transformed into isotropic state at about 148 ℃, indicating that the mesophase range is very narrow. On cooling process from isotropic state of compound BI-CHO-4, distinct schlieren texture could be observed indicating that compound BI-CHO-4 could also self-assemble into smectic C phase (Fig. 1b).[16] The phase transition temperatures were further detected by DSC (Fig. 1c and 1d). In the DSC curve of compound BI-CN-4, two endothermic peaks at 140.5 ℃ and 170.5 ℃ could be observed in the second heating cycle which corresponded to the phase transition from crystalline state to crystalline state and from smectic C to isotropic state, respectively. Interestingly, there were two exothermic peaks at 89.7 ℃ and 144.8 ℃ which could be attributed to cold-crystallization transition peaks in the second heating cycle. Such exothermic peaks appeared in the second heating cycle might be due to the appearance of metastable structure cooling from isotropic state, which could transform into more stable well-ordered structures via endothermal recrystallization process (Fig. 1c). Only one exothermic peak at 131.9 ℃ in the first cooling cycle could be detected which corresponded to the phase transition from isotropic state to smectic C phase (Fig. 1c). In the DSC curve of compound BI-CHO-4, four endothermic peaks at 77.3 ℃, 127.8 ℃, 140.5 ℃ and 148.4 ℃ could be observed in the second heating cycle. These peaks at 77.3 ℃ and 127.8 ℃ could be ascribe to the phase transition from crystalline state to crystalline state. The peaks at 140.5 ℃ and 148.4 ℃ could be assigned to the phase transition from crystalline state to smectic C phase and from smectic C phase to isotropic state (Fig. 1d). In the second heating cycle, two exothermic peaks at 139.1 ℃ and 73.1 ℃ could be observed which corresponded to the phase transition from isotropic state to smectic C phase and from smectic C phase to crystalline state, respectively (Fig. 1d). The DSC data were well in line with the observations under POM further demonstrating that compound BI-CN-4 is monotropic liquid crystal whereas compound BI-CHO-4 is enantiotropic liquid crystal (Fig. 1a-1d). The difference in thermal behavior and self-assembly behavior of both mesogens could be attributed to the position of cyano group. The cyano group which is adjacent to the phenylpyrene unit could increase the molecular conjugation and finally lead to close molecular stacking. Therefore BI-CN-4 exhibited much higher melting temperature than that of BI-CHO-4 and BI-CHO-4 was more likely to form lamellar structure.
2.3. Solvatochromic properties
The solvatochromic behavior of the pyrene-based liquid crystals was examined in some common organic solvents with different polarities by using absorption and emission spectra (Fig. S1, Fig. 2 and Table S1-2). Both compounds BI-CN-4 and BI-CHO-4 display only one absorption band at about 345-359 nm and 362-370 nm, respectively and the absorption bands of BI-CN-4 and BI-CHO-4 display little transformation with the change in solvent polarities indicating that the electronic configurations in the ground state are very stable. Both compounds BI-CN-4 and BI-CHO-4 also display one emission band at 450 nm and 465 nm in non-polar cyclohexane, respectively. The fluorescence color of compound BI-CN-4 is relatively weak, in contrast, the fluorescence color of compound BI-CHO-4 is blue (inset of Fig. 2a-2b). The emission bands of compounds BI-CN-4 and BI-CHO-4 gradually red-shift, the fluorescence color of compound BI-CHO-4 gradually turn yellow-green, but the fluorescence color of compound BI-CN-4 is hardly detected due to the weak emission in solutions with the increase in solvent polarities (from cyclohexane to dimethyl formamide). These observations indicated that positive solvatochromic behavior could be achieved in BI-CN-4 and BI-CHO-4, which might be assigned to the intramolecular charge transfer. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) were obtained by density functional theory (DFT) to visualize the intramolecular charge transfer effect. The HOMOs of compounds BI-CN-4 and BI-CHO-4 were primarily situated on the pyrene unit and only a little situated on the cyanostilbene unit. On the contrary, the HUMOs of BI-CN-4 and BI-CHO-4 were primarily situated on the cyanostilbene unit and only a little were situated on the pyrene unit. By comparing the emission spectra of both BI-CN-4 and BI-CHO-4, compound BI-CN-4 exhibited more distinct solvatochromism behavior, which could be assigned to the more distinct charge transfer. The hole-electron examination was employed to explore the differences of charge transfer between BI-CN-4 and BI-CHO-4 by using Multiwfn (Fig. 2c).[34] The hole-electron distributions at the excited state demonstrated that the negative charges of BI-CN-4 and BI-CHO-4 primarily situated on cyanoethylene and the positive charges of BI-CN-4 and BI-CHO-4 primarily situated on pyrene. Therefore, both compounds exhibited distinct charge separation at the excited state. The Chole-Cele diagrams demonstrated that the charge centroids of BI-CN-4 and BI-CHO-4 were individual and the D indexes that represented the distance between the centroids of Chole and Cele showed big difference. The D indexes were simulated to be 6.61 Å for S0→S1 excitation of BI-CN-4 and 4.09 Å for S0→S1 excitation of BI-CHO-4, respectively (Fig. 2d). The big difference of D indexes indicated that different charge transfer after excitation for both compounds.[35] Therefore, a smaller distance between electron acceptor (cyano group) and electron donor (pyrene) had more greater effect on the charge transfer.
2.4. AIE and AIEE behaviors
Cyanostilbene a typical of AIE and AIEE materials on account of distorted conformation which could restrict intramolecular rotation. In order to explore the AIE and AIEE properties of BI-CN-4 and BI-CHO-4, the absorption and emission spectra in THF-H2O mixtures with varying proportions of water (fw: 0%-90%) were studied. The absorption spectra in pure THF exhibited two peaks at 278 nm and 345 nm for BI-CN-4 and also two peaks at 280 nm and 362 nm for BI-CHO-4 (Fig. S2). The position and intensity of absorption spectra displayed little change with increasing the fw from 0% to 90%. The level-off tails of the long-wavelength region emerged when the fw was higher than 30%, indicating that the aggregates were formed in the mixtures.[36] The emission spectrum of BI-CN-4 in pure THF exhibited an extremely weak band at 484 nm and the emission spectra displayed little fluctuation when the fw was lower than 30% (Fig. 3a-b). The position of the emission spectra displayed a little red shift due to the polarity and the intensity showed a little increase due to the formation of aggregates, when the fw was ranged from 30% to 70%. The emission intensity further increased with increasing the fw and finally reached to the maximum when the fw was 90% (Fig. 3a-b). In contrast, the emission spectrum of BI-CHO-4 in pure THF exhibited a strong band at 493 nm. The position of the emission spectra also showed a little red shift and the intensity showed a little decrease due to the increased polarity of the mixture, when the fw was ranged from 0% to 60% (Fig. 3c-d). The emission intensity further increased with increasing the fw and finally reached to the maximum when the fw was 90% (Fig. 3c-d). These observations in absorption and emission spectra in THF-H2O mixtures of BI-CN-4 and BI-CHO-4 demonstrated that BI-CN-4 and BI-CHO-4 exhibited AIE and AIEE behaviors, respectively.
2.4. Mechanochromic behaviors
The mechanochromic behaviors of BI-CN-4 and BI-CHO-4 in of the pristine, ground, and fumed states were also explored by the absorption and emission spectra, fluorescence lifetimes, powder X-ray diffraction analysis and differential scanning calorimetry curves. Both compounds BI-CN-4 and BI-CHO-4 display a blue-emission band at 472 nm and 474 nm in the pristine state, respectively (Fig. S3 and Fig. 4). The fluorescence lifetimes of BI-CN-4 and BI-CHO-4 were about 1.89 ns and 2.40 ns, respectively (Fig. 4e-f). After BI-CN-4 was ground in mortar for several minutes, the emission spectrum showed obvious red shift to 498 nm, the fluorescent color turned into blue-green and the fluorescence lifetimes showed little fluctuation to 1.87 ns (Fig. 4a, 4c and 4e). For BI-CHO-4, the emission spectrum showed more obvious red shift to 512 nm, the fluorescent color turned yellow-green and the fluorescence lifetimes showed obvious increase to 4.26 ns (Fig. 4b, 4d and 4f) after grinding in mortar for several minutes. After fuming with n-hexane for several minutes, the emission spectra of BI-CN-4 and BI-CHO-4 recovered to the original state. The reversible changes could be attributed to the change of molecular stacking and molecular conformation caused by the external mechanical force and further solvent fuming.
In order to explain the mechanism of mechanochromic behavior of BI-CN-4 and BI-CHO-4, powder X-ray diffraction and differential scanning calorimetry investigations were conducted. The powder X-ray diffraction patterns of BI-CN-4 and BI-CHO-4 in the pristine state clearly showed several intensive and sharp reflection peaks at 5°-40° indicating well-ordered molecular stacking (Fig. 5a-b). After BI-CN-4 and BI-CHO-4 were ground in mortar for several minutes, some characteristic peaks became weak and even disappeared demonstrating that the molecular stacking became disordered. Further, after fuming with n-hexane for several minutes, the peaks appeared again demonstrating that the well-ordered molecular stacking recovered by fuming (Fig. 5a-b). Additionally, the DSC curves of the pristine solid of BI-CN-4 and BI-CHO-4 for the first heating cycle only showed one endothermic peak at 173.4 ℃ and 150.7 ℃, respectively, which could be attributed the phase transition from solid state to liquid state (Fig. 5c-d). After BI-CN-4 and BI-CHO-4 were ground in mortar for several minutes, the DSC curves of the ground solid of BI-CN-4 and BI-CHO-4 for the first heating cycle appeared one new exothermic peak at 53.6 ℃ and 54.1 ℃, respectively, which could be attibuted to thermal crystallization peak, demonstrating that the amorphous state could be formed by grinding (Fig. 5c-d). Therefore, the change in molecular stacking and formation of amorphous state by grinding would be responsible for the mechanochromic behaviors.